Physical inactivity is a known risk factor for obesity and related chronic diseases, including cardiovascular disease and certain cancers. Substantial variations exist in physical activity and obesity by various factors such as socioeconomic status and race/ethnicity. Recent research has clarified the multiple domains of physical activity (discretionary, household, transport-related and occupational) as well as the role that the built environment can play in supporting or impeding physical activity. It is now clear that transport-related physical activity, i.e. activity undertaken to get to a destination, can help play a major role in achieving recommended levels of physical activity. Public transit is a natural candidate for study in this regard because use of transit almost invariably requires some degree of activity to travel to and from the stops, such as walking or biking. The purpose of the proposed time-sensitive project is to evaluate, in the context of a natural experiment, the short and long-term effect of a new light rail transit (LRT) system on adult's physical activity in Houston, Texas. We are particularly interested in understanding this effect in the low-income, ethnically diverse adul population residing along the LRT line. This study is possible because of an ongoing expansion of the city's LRT system. To accomplish this goal, we propose the following three specific aims in 1,400 residents living within a half-mile of the new rails lines and 500 living beyond walking distance of the lines: (1) Evaluate the short-term (<12 months) and long-term (24 & 36 months) impact of accessibility to a new LRT line on LRT and overall transit use; (2) Evaluate the short- and long-term impact of LRT use on transit-related and overall physical activity levels; and (3) Using travel choice modeling techniques, examine the factors (such as demographic, socioeconomic, and environmental factors) influencing individuals' LRT mode choice decision, as well as the relation between individuals' LRT mode choice and active-travel behavior choices. We will assess typical travel patterns and transit use with travel diaries; self-reported physical activity; measure resident perceptions of the neighborhood environment; conduct on-the-ground neighborhood audits; and develop GIS-based profiles of the built environment of study neighborhoods. A subgroup of 1000 participants (500 each in intervention and control areas) will wear accelerometers for one week to provide a direct measure of physical activity. Baseline data will be collected prior to the lines' opening in late December 2013, with follow-up occurring once a year for three years. This proposed project improves substantially upon related prior studies. First, with a follow-up of three years post-LRT opening, we will be able to determine sustainability of changes to physical activity. Second, our target population is primarily minority and lower income, a collectively understudied group regarding physical activity. The population is also representative of demographic changes projected to occur in the rest of the Unites States. Third, we have a strong interdisciplinary team of consisting of specialists in epidemiology and public health, biostatistics, behavioral science and transportation engineering.